Field of Invention
The present invention relates to a valve train layout structure, and more particularly, to the valve train layout structure including a return spring and a camshaft-in-camshaft.
Description of Related Art
An internal combustion engine generates power by flowing fuels and air into combustion chambers and combusting them.
For combustion, an intake valve is opened by a driving camshaft and while the intake valve is opened, the air flows into the combustion chamber.
Further, an exhaust valve is opened by the driving camshaft after combustion and while the exhaust valve is opened, combustion gas is discharged out of the combustion chamber.
Optimum operation of an intake valve and an exhaust valve is adjusted depending on rotating speed of an engine.
This is because adequate valve lift or valve opening/closing time varies depending on the engine rotation speed.
Like this, the way of varying opening or closing time of an intake valve or an exhaust valve in accordance respectively with low speed or high speed of an engine in order to supplement the general drawbacks thereof is called variable valve timing (VVT) method.
Unlike a prior camshaft, a camshaft-in-camshaft comprises a hollow camshaft, namely an outer shaft and a different shaft inserted therein, namely an inner shaft.
There are two kinds in cam lobes of a camshaft-in-camshaft, one of which is a first cam fixed to an outer shaft and the other of which is a second cam fixed to an inner shaft and rotatable on the outer shaft.
A camshaft-in-camshaft structure has been devised so that among two types of valve connected thereto, a first type of valve is moved invariably in line with engine timing without special control and the movement of a second type of valve is controlled in order for the phase of the valve to become different from that of the first type of valve.
A cam phaser is the control apparatus which varies a phase between a first cam and a second cam.
By using the camshaft-in-camshaft and the cam phaser, continuous variable valve timing (CVVT) method can be realized.
A camshaft-in-camshaft in which a cam phaser varies a phase between a first cam and a second cam and thereby varies a phase between two valves is generally called a control camshaft.
It is generally the case that a cam phaser is mounted directly to a control camshaft to advance or retard (hereinafter, vary) an intake or an exhaust valve timing.
However, it occurs that a cam phaser can't be mounted directly to a control camshaft on account of a layout structure when an engine is mounted in a vehicle.
To overcome this, a major alteration for parts limiting the layout structure is needed. However, such an alteration is so big task that not only an engine system but also total package system of a vehicle must be changed. It comes close to a development project of a new engine.
In case of a modified engine, to cope with the above problem is practically almost impossible.
Accordingly, an alteration of a structure, a mounting place or a mounting method of a cam phaser is required and researches on this issue are being conducted.
In the meantime, when engine starting is off, a control camshaft generally stops with retarded valve timing on account of structural properties and inertia of a cam phaser.
As a result, a problem of unstable initial combustion happens because too much swirl is generated in cold starting of an engine.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.